Animation Nodes: Wait x seconds, output 0, then output random number (for input to object instancer)
You can multiply the output of the Random Number Node with a value that is one for all frames except those that are multiple of $x$. We can detect if a value is a multiple of $x$ using modulo as follows:



Th result of the previous node tree is as follows:



The number of objects briefly goes to zero and change into a random number every 10 frames.